Question: Why the figure is rougher in implicitplot when I increase the gridrefine?

 I'm new to Maple, recently I'm focusing on the implicitplot.  "Help" tells me that gridrefine specifies the number of recursive subdivisions to perform on any cells in the initial grid that have been determined to contain part of the curve to be plotted. Now I have a function to implicitplot. I find that when I increase the gridrefine, the figure is rougher, why does this happen since the recursive subdivisions are increased?

restart: F:=(x,y) -> -(1/2)*(25*(-2/25+y^2*(x^2+1)))*(1/2)^((-x^2+1)/(x^2+1))+4*(1/2)^(2*x^2/(x^2+1))*(1/25)+5*y^2*(1/2)^(-2*x^2/(x^2+1)); Digits:=20; plots:-implicitplot(F,0..20,0..10,gridrefine=3,crossingrefine=5,numpoints=1000,labels=["x","y"]); plots:-implicitplot(F,0..20,0..10,gridrefine=6,crossingrefine=5,numpoints=1000,labels=["x","y"]);